Human .beta.-endorphin has been isolated in a highly purified state from human pituitary glands. It is identical with the COOH-terminal 31-residue part of the human .beta.-lipotropin. The structure of human .beta.-endorphin has been confirmed by solid phase synthesis. See in this regard Li et al., J. Med. Chem. 20, 325-328 (1977) and U.S. Pat. No. 4,038,222. .beta.-endorphin has been found to be a potent analgesic when administered directly into the brain and assayed in the tail-flick, hot-plate and writhing in mice and in the wet shake test in rats. On a molar basis, .beta.-endorphin is 18-33 times more potent than morphine and its actions are blocked by naloxone. By intravenous administration .beta.-endorphin produces 3-4 times more potent effects than morphine. When administered i.c.v., it mimicks morphine in almost all of its effects, including causation of prolonged cataleptic states, development of tolerance and dependence, respiratory side effects and stimulation of release of somatotropin and prolactin in rats.
Previous synthesis of .beta.-endorphins have been carried out by solid phase methods. A solution synthesis offers the advantage of more efficient scale-up to allow production of amounts needed for clinical evaluations and ultimately commercial production of this compound.